The study in the rabbit of the serological systems which are under gene control continues to be the major subject of our investigations. We have in our colony seven blood group loci controlling 21 red cell antigens, one histocompatibility locus with 10 alleles, an Ir locus, an MLC locus and 5 serum protein loci which serve as the basis for our work in isoimmunization, transplantation and the genetics of the immune response. We have continued our work on the gene-antigen relationhip as manifested in the qualitative difference between interaction products of alleles. With the number of serological loci defined in our colony we have begun the study of the formal genetics of the rabbit.